1. Field of the Invention
This invention relates to a display unit such as a liquid crystal display. Particularly, the invention relates to controlling display voltages generated by a drive circuit.
2. Description of the Related Art
Recently, electronic devices such as a telephones or personal computer have began to incorporate device a display. Particularly, portable devices such as note-book type personal computers, portable telephones unit or portable television have become quite popular now. Each of these types of portable devices may include a liquid crystal display (hereinafter called "LCD").
The LCD has a LCD drive circuit to drive a display portion as display circuit. The LCD drive circuit generates a plurality of display voltages that are used by the display circuit. These display voltages are generated by activatting a booster circuit. For example, the booster circuit generates these display voltages based on a supply voltage supplied by a supply voltage source.
The booster circuit is popular to charge a pump system. For this case, the booster circuit has a capacitors store electrical charges used to charge a voltage source of the display circuit. These capacitors are coupled between corresponding wires and a reference voltage source such as a ground voltage source. Each of the wires transmit a correponding one of these display voltages generated by the booster circuit.
For the LCD drive circuit, when the supply voltage VDD decreases rapidly, electrical charge stored the capacitor does not discharge. Therefore, the wires leave electrical charge stored for a long time. As a result, the display portion of the display circuit leaves display. Thus, there occur instances in which information is displayed carelessly for viewing by a third party and there is a reduction in the life of the display circuit and malfunction or the like occur at restart-up, etc.
Particularly, when the portable device is in operation, the battery is removed without interrupting the power supply by a switch and the charged electrical charge is discharged. Therefore, it is necessary for the portable device to have the above problems solved.
For solving these problems, the solution measures further require more reliable operation when the supply voltage VDD is reduced.
The solution measures further require that there be no interference with a reduction in the size and cost of the display device itself.